Chemical-mechanical polishing has been used for many years as a technique for polishing optical lenses and semiconductor wafers. More recently, chemical-mechanical polishing has been developed as a means for planarizing intermetal dielectric layers of silicon dioxide and for removing portions of conductive layers within integrated circuit devices as they are fabricated on various substrates. For example, a silicon dioxide layer may cover a metal interconnect conformably such that the upper surface of the silicon dioxide layer is characterized by a series of non-planar steps corresponding in height and width to the underlying metal interconnects.
The step height variations in the upper surface of the intermetal dielectric layer have several undesirable characteristics. Such non-planar dielectric surfaces may interfere with the optical resolution of subsequent photolithographic processing steps, making it extremely difficult to print high resolution lines. Another problem involves the step created in the coverage of a second metal layer over the intermetal dielectric layer. If the step height is relatively large, the metal coverage may be incomplete such that open circuits may be formed in the second metal layer.
To combat these problems, various techniques have been developed to planarize the upper surface of the intermetal dielectric layer. One such approach is to employ abrasive polishing to remove the protruding steps along the upper surface of the dielectric layer. According to this method, a silicon substrate wafer is mounted face down beneath a carrier and pressed between the carrier and a table or platen covered with a flat pad continuously covered with a slurried abrasive material (in FIG. 1, a prior art abrasive disk and holder are shown in the same position as such a wafer and carrier).
Means are also provided for depositing the abrasive slurry on the upper surface of the pad and for forcibly pressing the substrate wafer against the polishing pad, such that relative movement of the platen and the substrate wafer relative to each other in the presence of the slurry results in planarization of the contacted face of the wafer. Both the wafer and the table may be rotated relative to each other to rub away the protruding steps. This abrasive polishing process is continued until the upper surface of the dielectric layer is substantially flat.
Conventional polishing pads may be made of a uniform material such as polyurethane or may be formed from multilayer laminations having non-uniform physical properties throughout the thickness of the pad. Polyurethane polishing pads are typically formed by reacting the reagents that form polyurethane within a cylindrical container. After forming, a cylindrically shaped piece of polyurethane is cut into slices that are subsequently used as the polishing pad. A typical laminated pad may have a plurality of layers, such as a spongy and resilient microporous polyurethane layer laminated onto a firm but resilient supporting layer comprising a porous polyester felt with a polyurethane binder. Polishing pads typically may have pores that have a size of about 100-200 microns.
Conventional polishing pads typically may also have microtextured surfaces resulting from a microscopic bulk texture of the pad resulting from factors intrinsic to the manufacturing process. Some of the factors which influence the microscopic bulk texture of a conventional pad are the nature or texture of the work surface, such as waves, holes, creases, ridges, slits, depressions, protrusions, gaps or other spaces, and the size, shape and distribution frequency or spacing of such features. Since polishing does not normally occur across the entire pad surface, the microtexture of the pad or any macrotextures made by surface machining, may only be formed into the portion of the pad over which polishing is to take place.
During the polishing process, the material removed from the wafer surface and the abrasive, such as silica, in the slurry become embedded in the pores and other free spaces within the microscopic and macroscopic bulk texture of the polishing pad at and near its surface. One factor in achieving and maintaining a high and stable polishing rate is providing and maintaining the pad in a clean condition. Pad cleaning or reconditioning is a technique whereby the pad surface is returned to a proper state for subsequent polishing work. The purpose of the cleaning method selected is to remove the contaminating particles of wafer, abrasive or other debris from the free spaces or interstices at or slightly below the pad surface. Prior art cleaning techniques include diamond grit conditioners that grind away the contaminated layer of the pad, thereby removing the contaminants along with a portion of the pad material itself.
FIG. 1 is an illustration of using a diamond grit conditioner to clean a polishing pad 10 attached to a platen 14. Above the polishing pad 10 is a holder 12 for carrying a diamond grit disk 11 and pressing it against the contaminated face 13 of the polishing pad 10. During reconditioning, the carrier 12 is rotated by drive shaft 15 and the platen 14 is rotated by drive shaft 16. The carrier 12 and the platen 14 may rotate either clockwise or counterclockwise, but typically the carrier 12 rotates in the same direction as the platen 14. While the carrier and platen are rotated, the carrier 12 may be oscillated back and forth across the polishing pad as indicated by the arrow O. In this direct contact reconditioning technique, the abrasive disk 11 and the carrier 12 are essentially substituted for the respective wafer and its carrier mentioned in the prior art planarization technique also described above.
Obviously, such direct contact processes significantly reduce the life of the polishing pad. To avoid such degradation of the pad, other prior art techniques include using a water knife or an air knife in an attempt to blow away the contaminants. However, the water or air velocities required for effectiveness are such that substantial portions of the pad adjacent to its surface may also be removed or damaged.
It has also been determined that ineffective reconditioning of the polishing pad has a direct correlation to the amount of scratching caused to a wafer surface by polishing with the poorly reconditioned pad. This in turn causes significant losses in the yield of semiconductor wafers. Thus, the consistency and quality of polishing pad reconditioning by prior processes has presented two problems. The first of these is the reduction in polishing pad life due to the destructive nature of the reconditioning techniques previously available. Secondly, the poor quality of polishing pad reconditioning provided by prior art processes has resulted in a relatively high loss of product yield due to wafer damage caused by the reconditioning technique. Therefore, there is a need for a polishing pad reconditioning process of higher quality and consistency than provided by prior art processes.